1. Field of the Invention
The present invention relates to methods and means incorporating electrostatic precipitators for cleaning air and industrial and domestic gases contaminated with ecologically noxious substances. More particularly, it provides an improved method and highly efficient electrical apparatus for that purpose.
2. Prior Art
In the prior art, gas cleaning is usually carried out in electrical precipitation reaction chambers in which contaminated gases flow through an electrode system.. The electrode system of such electrical precipitators is activated by short, high voltage electrical pulses superimposed on a constant, high, direct-current voltage. The pulses give rise to so-called streamer corona discharge within the reaction chamber causing a flow of high-speed electrons which activate or ionize gas molecules to produce non-noxious aerosols and solid particles within the chamber. The aerosols and solid particles are removed from the gases in the reaction chamber by the electrostatic conductive field created by the constant direct-current voltage existing between the electrodes of the electrode system.
The efficiency of the cleaning process depends on the density and the energy of the electron flow generated by the streamer corona discharge. Both of these parameters rise with an increase in pulse amplitude and with an increase in the steepness of the pulses. The pulse steepness is an important factor because the pulse amplitude that can be achieved without electrical breakdown of the inter-electrode space in the reaction chamber is a function of pulse steepness.
Based on these considerations, it is an objective of the present invention to provide an improved waste stream cleaning method and apparatus which embody a high voltage power supply capable of generating voltage pulses of the high amplitude and steepness required for producing a high density energy flow within a precipitation reaction chamber, and also of producing a constant, high, direct-current voltage on which the pulses are superimposed.
Another object is to provide a power supply for use with such a method and apparatus which is extremely efficient from an energy consumption point of view so that the apparatus may operate normally for long periods of time.
A further object of the invention is to provide a precipitation reaction chamber for such use which includes a high voltage corona-producing electrode that responds to the high voltage pulses from the power supply to generate a pulse streamer corona of sufficiently high current to enable the transfer of the required amount of electrical power into the gases in the precipitation chamber so as to activate or ionize the gas molecules to produce non-noxious aerosols and solid particles.
A still further object is to provide a reaction chamber constructed to permit the application without electrical breakdown of a constant direct-current voltage from the power supply which is sufficiently high for the removal of the resulting non-noxious aerosols and solid particles from the gases flowing through the reaction chamber.
There are two main groups of prior art apparatus for cleaning waste stream pollutants by means of electric discharge in a precipitation reaction chamber. A first group involves the use of a pulse transformer for the generation of high voltage pulses. Typical examples of the prior art apparatus of the first group are described in U.S. Pat. Nos. 4,016,060 and 4,808,200. A second group of prior art apparatus involves the use of a direct discharge by appropriate switching means of a power storage unit, such as a capacitor or a pulse-forming L-C line, to produce the high voltage pulses. Typical examples of the prior art apparatus of the second group are described in U.S. Pat. Nos. 4,183,736 and 4,713,093.
A disadvantage of apparatus using a pulse transformer, such as the apparatus of the first group, is the necessity of providing two separate power supplies, one for producing the constant direct-current high voltage for the reaction chamber, and a second for energizing the pulse-generating system, the second power supply being connected to the primary coil of the pulse transformer. Another disadvantage of the prior art apparatus of the first group resides in the fact that a pulse transformer capable of generating voltage pulses of the amplitude, power, duration and frequency required for producing a streamer corona is a very complicated and expensive device.
A disadvantage of the prior art apparatus of the second group is the impossibility of obtaining a pulse voltage with an amplitude greater than double the charge voltage. These devices are incapable of producing electrons with sufficiently high energy to perform an efficient gas cleaning operation in the reaction chamber.
Another disadvantage of the second group of prior art devices stems from the requirement that in order to avoid short circuits of the power supply, the power storage unit must be isolated from the power supply by a resistor or an inductance coil having high resistivity. However, in the periods when the capacitor in the power storage unit is being charged, the resistor or inductance coil dissipates as much power as is stored in the power storage unit, resulting in low efficiency.
Still another disadvantage inherent in the prior art apparatus of the second type lies in the fact that due to high resistivity of the inter-electrode space in the reaction chamber, only a part of the power stored in the power storage unit is inserted into the gas flowing through the reaction chamber. The remaining part of the power is dissipated in the resistivity of the overall circuit. This results in a further reduction in efficiency.
From the foregoing, it will be appreciated that another object of the subject invention is to provide a method and means for removing pollutants from a gas or liquid waste stream which avoid or overcome the various previously mentioned disadvantages of the prior art electrostatic cleaning methods and precipitators.
It is well known in the prior art that various processes for the removal of ecologically harmful substances from waste gas and liquid streams can be carried out or enhanced by treating the streams with conventional oxidizing agents, such as ozone. Certain prior art electrostatic precipitators are designed to produce ozone to enhance the oxidation process.
The efficiency of the oxidation of the pollutant molecules depends on their chemical structure and concentration and on the reactance of the particular oxidant or oxidants. By way of comparison, the reaction rate constants K.sub.298 for several standard reactions involving typical inorganic and organic industrial pollutants in the presence of the most common oxidant-reagents at room temperature are set forth in the following Table 1.
TABLE 1 ______________________________________ Reaction K.sub.298, cm.sup.3 /molecule .multidot. sec ______________________________________ O.sub.3 + SO.sub.2 .fwdarw. SO.sub.3 + O.sub.2 &lt;8 .multidot. 10.sup.-24 OH + SO.sub.2 .fwdarw. HOSO.sub.2 1.1 .multidot. 10.sup.-12 HO.sub.2 + SO.sub.2 .fwdarw. HO + SO.sub.3 &lt;1 .multidot. 10.sup.-18 O.sub.3 + NO .fwdarw. NO.sub.2 + O.sub.2 1.8 .multidot. 10.sup.-14 OH + NO .fwdarw. HONO 6.7 .multidot. 10.sup.-12 HO.sub.2 + NO .fwdarw. NO.sub.2 + OH 6.6 .multidot. 10.sup.-12 O + NO .fwdarw. NO.sub.2 1.9 .multidot. 10.sup.-12 O.sub.3 + NO.sub.2 .fwdarw. NO.sub.3 + O.sub.2 3.2 .multidot. 10.sup.-17 OH + NO.sub.2 .fwdarw. HNO.sub.3 1.1 .multidot. 10.sup.-11 O + NO.sub.2 .fwdarw. NO + O.sub.2 5 .multidot. 10.sup.-12 O.sub.3 + R-CH.dbd.CH.sub.2 .fwdarw. products 10.sup.-18 -10.sup.-17 O + R-CH.dbd.CH.sub.2 .fwdarw. products 10.sup.-13 -10.sup.-11 OH + R-CH.dbd.CH.sub.2 .fwdarw. products 10.sup.-12 -10.sup.-11 ______________________________________
Experience has demonstrated that many of the harmful and noxious substances most frequently encountered in the modern industrial environment tend to display a relatively low rate of reaction with ozone. Analysis of the K.sub.298 values set forth in Table 1 confirms that the reactions involving ions or radicals (O, OH, HO.sub.2) are much more rapid than those of the same pollutants with ozone. To take advantage of these phenomena, it is another object of the present invention to provide an effective method and electrical apparatus for producing primarily highly active intermediate substituents (e.g., O, OH, HO.sub.2 ions), rather than ozone.